On recently buying some well-rotted stable manure for my garden, I was naturally apprehensive lest it be too obviously jumentous. I’m glad to be able to report that my worries were unfounded.
The word is usually explained as suggesting that something is smelling like the urine of a horse. It comes from Latin jumentum, which the Oxford English Dictionary explains means a yoke-beast, from jugum, a yoke. Though this might reasonably include oxen, the Oxford Latin Dictionary helpfully notes — somewhat surprisingly in view of its origin — that in Roman times it usually meant horses or mules, not cattle. Similarly, the obsolete English word jument, from the same source, could mean any beast of burden, but was most often applied to a horse or donkey. In French, jument means a mare.
The first appearance of jumentous that I can trace is in this report of the symptoms of a sick person:
No motion of the bowels; urine very scanty, red with a jumentous and lateritious sediment, also great thirst, great dryness of mouth and tongue, which were covered with a dirty white covering.
The British Journal of Homoeopathy, 1801. The word was deemed to be unfamiliar enough that it was defined in a footnote as relating to a working horse. Lateritious, from Latin later, a brick, means resembling brick, or coloured brick-red, a word that has usually been applied only to urine.
Other nineteenth-century works used jumentous in the same way, but by the end of the century it had become extremely rare, and remains so. Peter Bowler asked of it in The Superior Person’s Second Book of Weird and Wondrous Words of 1992, “Is this word really necessary?” You may concur; I couldn’t possibly comment.